Conventionally, a PLL circuit is generally utilized in a phase modulation apparatus used in a communication device. Requirements of a phase modulation method used by such a PLL circuit include low cost, low power consumption, good noise characteristics, and high modulation precision. In order to increase modulation precision, it is desirable for the PLL frequency bandwidth (hereinafter referred to as “PLL bandwidth”) to be made wider than the frequency bandwidth of a modulation signal (hereinafter referred to as “modulation bandwidth”) in the phase modulation method of such a PLL circuit. However, since noise characteristics degrade when the PLL bandwidth is widened, the result is that it is difficult to widen the modulation bandwidth and perform wideband modulation.
Thus, a 2-point modulation method has been proposed whereby the PLL bandwidth is set narrower than the modulation bandwidth, and modulation within the PLL bandwidth and modulation outside the PLL bandwidth are performed at two different places (see, for example, Patent Document 1). FIG. 1 is a block diagram showing the configuration of a phase modulation apparatus according to the prior art proposed in Patent Document 1. As shown in FIG. 1, the configuration of a phase modulation apparatus using a 2-point modulation method such as proposed in Patent Document 1 comprises a PLL circuit 20 that includes a reference oscillator 21, a limiter 22, a reference frequency divider 23, a phase frequency detector 24, a charge pump 25, a loop filter 26, an adder 27, a VCO (Voltage Controlled Oscillator) 28, and a frequency divider 29; as well as a modulator 30, an adder 31, a constant F 32, a delta sigma modulator 33, an adder 34, a constant P 35, a charge pump scaling section 36, and a modulation scaling section 37.
In FIG. 1, VCO 28 of PLL circuit 20 outputs an RF phase modulation signal of a frequency in accordance with the voltage input to the control voltage terminal of VCO 28. Frequency divider 29 divides the frequency of the RF phase modulation signal output from VCO 28. Phase frequency detector 24 compares the phase of the signal output from frequency divider 29 with the phase of a reference signal output from reference frequency divider 23, and outputs a signal in accordance with the phase difference. Loop filter 26 averages the output signal from phase frequency detector 24.
Modulation amplitude scaling section 37 outputs a modulation signal from adder 27 to VCO 28 based on modulation data. Charge pump scaling section 36 controls residual modulation within the phase locked loop by controlling charge pump 25, and thereby enables modulation to be output more accurately. That is to say, the phase modulation apparatus in FIG. 1 generates an RF phase modulation signal by supplying control signals from two points—charge pump scaling section 36 and modulation amplitude scaling section 37. In this way, a phase modulation apparatus using a 2-point modulation method makes it possible to implement multimode phase modulation covering the range from narrowband to broadband. Patent Document 1: Unexamined Japanese Patent Publication No. 2003-510899